It is routine practice in modern hospitals to take blood samples from patients by means of one or more evacuated test tubes placed in succession inside a holder having a two-pointed cannula; one pointed end of the cannula pierces the vein of the patient and the other pointed end pierces the stopper of the evacuated test tube thus providing a flow of blood from the vein to the evacuated tube.
Holders for vacuum tubes are however relatively expensive and therefore non-disposable as well as bulky devices subject to contamination (see U.S. Pat. No. 3,520,292). Also, when double pointed cannulas are not a built-in feature of holders they have to be joined to them by an operator; this process of handling double pointed cannulas is hazardous since the operator may pierce his fingers or contaminate the cannula. Moreover, double pointed cannulas do not easily allow for injection of soluble drugs into the vein, whereas in medical practice it is sometimes desirable to inject drugs by the cannula into the vein after having drawn blood, in this way avoiding the attendant pain of piercing a new vein. Another drawback when using double pointed cannulas is that sometimes blood flushes back from the vein through the cannula to its inner end when withdrawing the tube thus spilling blood over the surroundings. Most important, when using vacuum tubes with a double-ended cannula if the outer tip lodges within tissues rather than a vein, extravasation of blood from the adjacent vein and hematoma may result when the evacuated tube is pierced by the outer tip of the cannula. It is then necessary for the operator to withdraw the cannula as quickly as possible. Even then, the operator may not recognize the situation quickly enough because the full difference in pressure is applied to the outer tip of the cannula as the stopper of the evacuated tube is pierced.
Further, the conventional device has a holder member surrounding the proximal end of the double ended cannula for reception of the stoppered end of the evacuated tube. The diameter of the member is large enough so that the cannula enters the skin at a relatively sharp angle increasing the danger of passing through the further wall of the blood vessel so that the tip of the cannula is within the tissue rather than within the blood vessel. Also, when an evacuated contained is not in position over the proximal end of the cannula clotting at the tip may occur thus interfering with the operation of the device when the taking of an additional specimen is desired.